The present invention relates to a heat sealable wrapping or packing film which is capable of forming a peelable seal. The seal is achievable either between two films or sheets of this kind, or between one film or sheet of this kind and a substrate. Depending on which substrate is used, an additional tie layer adhesive may or may not be needed between this film or sheet and the substrate.
A peelable seal is defined to be the seal or joint between two films produced by heat sealing or impulse sealing, the joint thus formed having the property of being able to open in the original plane of joining of the two films by the action of a pulling force, without wrenching off or tearing occurring in the material of the two films used to make up the joint. For the purposes of the present invention, the peelable seal must possess a mechanical resistance sufficient to maintain the wholeness and the tight-seal properties of the packing and wrapping during storage and transport until such time as the packing or wrapping is opened by the user of the article. The mechanical resistance of the peelable seal must be low enough to permit ready manual opening of the joint, i.e., without the use of any auxiliary instrument.
In the past, many varieties of thermoplastic materials have been employed in the manufacture of films capable of forming peelable seals. See, for example, U.S. Pat. No. 4,189,519, to American Can, which discloses a blend for producing a peelable heat seal comprising (1) about 50 to 90 percent by weight of a copolymer of about 80 to 96 percent by weight ethylene and about 4 to 20 percent by weight of an ethylenically unsaturated ester, and (2) about 10 to 50 percent by weight of a crystalline isotactic polybutylene. The film of '519 discloses films using ethylenic components containing 4 to 20 percent by weight of an ethylenically unsaturated ester.
U.S. Pat. No. 3,900,534 to Mobil Oil Corporation generally discloses thermoplastic shrink films with good heat seal characteristics and good optical properties, however, '534 does not address the need for a peel seal film.
U.S. Pat. No. 3,817,821 to E. I. DuPont de Nemours & Co. discloses blends of EVA with polybutylene at 20-40 weight percent.
U.S. Pat. No. 3,879,492 to UCB S. A. Belgium discloses blends of polybutylene+styrene-butadiene copolymer+LDPE+HDPE+polyisobutylene.
U.S. Pat. No. 4,539,263 to E. I. DuPont de Nemours & Co. discloses peel seals based on blends of propylene copolymers.
U.S. Pat. Nos. 4,665,130 and 4,666,778 discloses blends of polybytylene+EVA (or polyethylene)+polypropylene with polypropylene being less than 15 weight percent in the blend.
U.S. Pat. No. 4,359,544 to Shell Oil Company describes a method for promoting crystallization of various butene-1 homopolymers and copolymers, wherein the copolymers have no more than 15 mole% ethylene, wherein the method comprises adding to a composition comprising the butene-1 homopolymers or copolymer 0.02 to 5% wt stearamide and about 0.02 to 5% wt high density polyethylene.
U.S. Pat. No. 4,440,899 to Stamicarbon describes a polyolefin composition comprising an intimate mixture of 20 to 99.5% wt of a first stabilized polyolefin powder and from about 0.5% to about 80% by weight of a second at most slightly stabilized polyolefin powder wherein the second powder has a crystalline melt point at least one degree higher than that of the first powder and the second powder has the ability to be oxidized under process conditions. Both the first and second powders are polyolefins, and in particular a polyethylene composition is disclosed throughout the examples. The polyethylene so disclosed is generally one having a density of 0.938 g/ml.
U.S. Pat. No. 4,489,034 to Shell Oil Company describes a process using crystalline polymer sheets prepared by the peroxide reacting of a mixture of a polyalphaolefin selected from polypropylene and polybutylene with a polyethylene. In particular, this patent claims a product obtained by contacting a mixture of 95 to 65% wt of a mixture of about 50 to 96% wt of crystalline isotactic polyalphaolefin selected from the group consisting of polypropylene and polybutylene and about 50 to 40% wt of a polyethylene; 5 to 35% wt of an impact modifier; and an effective amount of 1,2-polybutadiene cross-linking efficiency improver.
U.S. Pat. No. 4,657,982 to DuPont Canada Inc. describes blends and films made from 80 to 99% of a polyethylene, 0.5 to 10% of a polybutene having a number average molecular weight of from 500 to 1500 and 0.5 to 10% wt of a polybutene having a number average molecular weight of from 1700 to 10,000.
U.S. Pat. No. 3,733,373 to Eastman Kodak Company claims a blend comprising about 70 to 99.9% wt of a substantially crystallizable polymer of 1-butene selected from homopolymers of 1-butene and copolymers of 1-butene and a polymerizable dissimilar olefin of 2-12 carbon atoms with the alpha-olefin being present in the copolymer at a concentration of up to 20% wt as well as second polymer, having 0.1 to 30% wt of a polymer of ethylene selected from a homopolymer of ethylene or a copolymer of ethylene and a higher alpha-olefin with the higher alpha-olefin being present in the copolymer in a concentration of up to about 10% wt and wherein the ethylene has a density of at least 0.93.
There has been a long felt need for new compositions, film or sheets and laminar structures usable as wrapping or packaging materials, having easy peelability at the seal and yet which will bond to polypropylene without the aid of an adhesive and will bond to other substrates using a tie layer adhesive having a clean sealing surface with few angel hairs contained therein upon opening of a sealed area.